142 Weed and Pii'sson — Bearjpaio Mountcdns^ Montana. 



the rock recalls strikingly certain monchiqnites described by 

 Kemp"^ in the Lake Charaplain district. It differs of course 

 from these in the character of the base. In polarized light 

 this colorless base breaks up into patchy, irregular, allotrio- 

 morphic areas, with gray polarization colors. Some sections 

 remain dark during a revolution and in convergent light these 

 furnish the faint, negative, uniaxial interference ligure of 

 nephelite. Other sections show the exit of a bisectrix and 

 that a biaxial mineral, a feldspar, is also present ; these present 

 at times the appearance of twinning according to the carlsbad 

 law, no albite twinning being seen. Careful study of the sec- 

 tion shows that the feldspar is not a plagioclase but an alkali 

 feldspar, which in all probability is anorthoclase from its associ- 

 ation with nephelite, but the optical means at present available 

 are not decisive in separating this feldspar from sanidine. 



Occasionally, with high powers, a rounded area surrounded 

 by minute grains of pyroxene and iron ore which define it may 

 be found ; they are of a pale brownish color, and are an iso- 

 tropic mineral which commonly appears faintly birefractive 

 from partial zeolitization. It is referred to sodalite. The 

 rock powder treated with dilute nitric acid gelatinizes abun- 

 dantly, confirming the presence of the nephelite ; the solution 

 gives a strong reaction for chlorine, confirming the presence 

 of the sodalite ; there is no reaction for sulphates, thus exclud- 

 ing hauyn and nosean.f 



From the description given it is evident that this rock is a 

 nephelite hasalt. It does not, it is true, present the black 

 appearance and dense texture so characteristic of the nephelite 

 basalts common in central Europe, but appears like a compact, 

 fine-grained intrusive ; on the other hand, it does not show the 

 coarse-grained character indicative of the truly abyssal rocks. 

 It is evidently closely allied to the lamprophyre group and 

 shows certain analogies to the absarokite of Iddings.J Since, 

 however, in mineral composition and in appearance, both 

 in the hand specimen and in thin section, it is most closely 



* Bull. 107, U. S. Geol. Surv., 1893. 



f In this connection it may be worthy of notice that rocks containing an appre - 

 ciable amount of nephelite may be powdered and safely tested for gelatinization 

 in the presence of olivine, if very dilute nitric acid is used. By experiment it 

 has been found that pure, finely-powdered nephelite dissolves easily and rapidly 

 in a solution so weak as one part of pure concentrated nitric acid in forty parts 

 of water, yielding gelatinous silica upon evaporation. Thus in the present case 

 a bulk of finely powdered olivine equal in amount to the rock taken, was treated 

 with very dilute acid under the same conditions. Upon boiUng a few moments 

 and then filtering, the filtrate upon evaporation yielded only a very small amount 

 of gelatinous silica, while the filtrate from the rock powder gelatinized abun- 

 dantly. Under these conditions the very small amount of olivine in the rock 

 could have yielded only an inappreciable amount of gelatinous silica. 



X Jour, of G-eol., vol. iii, No. 8, p. 935, 1895. 



